Electrolytic production of alkali and alkaline earth metals



Patented oer 13,1931

UNITED STATES- PATENT OFFICE.

JOHN J. GREBE AND RAY H. BOUNDY, OF MIDLAND, MICHIGAN ACSIGNOBS TO THEDOW CHEMICAL COMPANY, OF MIDLAND, MICHIGAN, A CORPORATION OF MICHIGANEDEC'IBOLYTIC PRODUCTION OF ALKALI AND ALKALINE EARTH No Drawing. V

The present invention relates to the production of alkali and alkalineearth metals, particularly to the production of metallic sodium ormetallic calcium, by the electrolysis of a fused bath of thecorresponding halide.

Although theoretically the electrolysis of a fused chloride shouldpermit production of the metal at a relatively highcurrent efii-'ciency, as compared with the electrolysis of a fused hydroxide as, forexample, in the case of sodium, yet in practice the expected highcurrent efficiencies have not been realized, and in the presentcommercial processes of the character in hand the current efficiency mayfall as low as% or lower.

In our experience with the electrolysis of a fused sodium chloride bath,as well as with a fused calcium chloride bath, initial currentefficiencies of high value were obtained, which, however, rapidlydecreased with continued operatiom'and in fact, although metal wasformed copiously at first, its formation of the various ways in whichthe in some cases ceased entirely after a short period. Thise phenomenonwas found to be caused by the presence in the bath of dissolved oxidesof the metal being formed by the electrolysis, such oxides resultingfrom the action of oxygen of the air which entered the cell with the'rawmaterial feed or through leakage, or otherwise. We have discovered thatthe deleterious effect of oxygen may be overcome by adding to the bath asubstance capable of uniting with the oxides present therein to forminsoluble compounds which are precipitated and thereby rendered inert.By means of our improved method a current efficiency as high as percent. or more may be regularly attained.

To the accomplishment of the foregoing and related ends the invention,then, consists of the steps hereinafter fully described and particularlypointed out in the claims, the following description setting forth butone principle of the invention may be carried out.

During the operation of an electrolytic cell for the production ofsodium, calcium or other alkali'or alkaline earth metals from theirfused halides, the action of the oxygen of the air which may enter withthe feed of Application filed m ,5, 1928. Serial No. 290,091,

raw material, b leakage or otherwise, re-' suits-in the pro uction ofoxides of the corresponding metals. in the fused bath, wherein theytendto accumulateas the electrolysis proceeds. We

have not determined precisel the nature of the influence exerted by theissolved oxides whereby the yield of metal is adversely affected, butthere seems to be a cyclic oxidation and reduction which dissipates apart of the Such oxides are soluble electrical energy supplied to the.cell. Such action is also aided, apparently, by the rapid diffusion ofthe several components of the bath at the operating temperature, whichwould be about 800 0., for example, in the electrolysis of fused sodiumchloride. Unless the aforesaid oxides are removed, the cumulative eflectof their increasing concentration in the bath may eventually consume thegreater part of the electrical energy without a corresponding yield ofthe desired product.

The oxides of the alkali and alkaline earth -metals are basic. incharacter, and we have found that at the temperature of the fusedelectrolyte they will unite readily with oxides of acidic character,such as alumina, silica, or boric acid, forming thereby aluminates,silicates, or borates, respectively, which are'in-' insolubleprecipitate.- For example, we have found that aluminum oxide willcompletely precipitate any sodium oxide or per-oxide that may be presentin a fused sodium chloride bath employed for the electrolytic productionof metallic sodium, forming an insoluble sodium aluminate which isprecipitated out. The same result is obtained, however, if,instead ofthe oxide, metallic aluminum or aluminum chloride be used. Working inthis way, we have found that a-continuous production of the desiredmetal ,is achieved at a satisfactorycurrent efliciency,

whereas, operating the same cell without employing a deoxidizer asherein disclosed, the

"oxide in the bath.

Since in practice great practical difliculties are connected with anattempt to prevent entirely the entrance oxygen into the cell,

our invention constitutes a valuable improvement in the art ofelectrolyzing a fused salt bath for the production of metals such assodium, calcium, or the like, of the alkali or alkaline earth metalgroups, concisely designated hereinafter as alkali-forming metals,

' wherein the action of oxygen affects the yield unfavorably. Ourimproved method may be applied with advantage to any type ofelectrolytic cell adapted to the process in hand, thereby counteractingthe harmful 'eifect of any oxygen which may enter, as well as makingless necessary the costly and burdensome devices for rendering the cellair-tight. Incidentally, the elimination of dissolved oxygen compoundsfrom the bath minimizes the consumption of oxidizable carbon or graphiteelectrodes, which are best adapted for use in a fused salt bath.

, In practice we have found it convenient to add the deoxidizer in theform of aluminum oxide along with the feed to the cell. The

' quantity to be added may vary within rather wide limits, dependantupon operating conditions, and the extent to which the design of thecell permits exclusion of air. However in general from one to two percent. of the weight of raw material fed will be sufficient under goodoperating conditions. It is best to maintain a slight excess ofdeoxidizer, which is itself insoluble in the bath and does notcausedisturbances, while at the same time it is capable of removingcompletely every trace of basic oxides dissolved therein. The sedimentor sludge which collects at the bottom of the cell is removed from timeto time by dipping, or by other suitable means.

We do not limit ourselves, however. to the manner of adding a deoxidizernor to the parlikewise silica, boric acid or other oxides of acidiccharacter.

In the following claims it is understood that the expression .an agentproviding-an oxide of acidic character includes such element itself, ora compoundthereof, and specifically that an agent containing aluminum.

includes metallic aluminum, or a compound thereof.

my invention may be employed instead of the one explained, change beingmade as re ards the method herein disclosed, provide the step or stepsstated by any of the following claims or the equivalent of such statedstep or steps be employed. y

We therefore particularly point out and distinctly claim as ourinvention 1. A method of the character described, which compriseselectrolyzing a fused bath containing chiefly a halide of analkali-forming meta-l, together with a minor component of at least oneof the following :a metal or compound of a metal capable of forming ametallic oxide of acidic character and fur ther capableof reacting withthe metal to be produced or with an oxide thereof to form whichcomprises electroyzing a fused bathcontaining chiefly a halide ofsodium, to-

gether with a minor component of at least one of the following, to wit:a metal or compound of a metal capable of forming a metal lic oxide ofacidic character and further capable of reacting with the metal to beproduced or with an oxide thereof to form a compound insoluble in saidbath.

3. A method of the character described, which comprises electrolyzing afused bath containing chiefly a halide of an alkali-form ing metal,together with a minor component of at least one of the followingaluminum or aluminum oxide.

4. A method of producing metallic sodium, which comprises electrolyzinga bath of the fused chloride thereof in the presence of an agentproviding-an oxide of acidic character.

5. In the electrolytic production of metallic sodium from a bath offused chloride thereof, the method of deoxidizing said chloride whichconsists in adding thereto an oxide of acidic character.

6. In the electrolytic production of metallic sodium from a bath offused chloride thereof, the method of deoxidizing said chloride whichconsists in adding thereto aluminum oxide.

7. The method of making an alkali-form- .ing metal which compriseselectroly zing a 8. Themethod of making metallic sodium. which compriseselectrolyzing a bath of fused chloride thereof while continuouslymaintaining said bath substantially free from oxides of sodium by addingthereto aluminum oxide.

9. The method of deoxidizing a fused halide of an alkali-forming metalcontaining oxide of such metal dissolved therein, which consists inprecipitating said oxide as an insoluble compound with an oxide ofacidic character. Other modes of applying the principle of" 10. Themethod of making an alkali-formmetal, which comprises electrolyzing abath of fused chloride thereof while continuously maintaining said bathsubstantially free from an agent containing aluminum.

11. The method of deoxidizing a fused dissolved oxides by adding theretohalide of an alkali-forming metal containing oxide of such metaldissolved therein, which consists in precipitating said oxide as aninsoluble compound with aluminum oxide.

12. The method of producing an alkali formin metal, which compriseselectrolyz'ing a bath 0 ing to free minum oxide thereto in admixturewith said fresh chloride.

Signed by us this 28th day of June, 1928.

JOHN J. GREBE. RAY H. BOUNDY.

